428 



method of experimentally adjusting the apparatus, making use 

 of Professor Wheatstone's differential resistance measurer, 

 by which I adjusted wires attached to the galvanometer, so 

 as to have the means of dividing a current, so that exactly 

 half any given current could be passed through the galvano- 

 meter coil. By this means I have proved that the indications 

 of my galvanometer are in the exact ratio of the currents 

 passing through it, at all events within the limits of the 

 powers of the batteries which I have tried it with. Thus, 

 if a current through the battery be found to balance 110 

 grains in the scale, the current from the same battery being 

 divided between the galvanometer coil and the diverting coil, 

 will, wdthin the limits of error arising from the three measure- 

 ments of wire, be found to lift 55 grains. 



Before entering on the consideration of the phenomena 

 of voltaic currents in relation to conduction, I feel it in- 

 cumbent upon me to draw your attention to the theory of 

 Ohm, which has been universally admired by those who 

 have studied it, and which has become the foundation of 

 the modern applications of mathematics to voltaic electricity. 

 I regret that I cannot at present go very fully into this, as 

 it would occupy too much time, that I have not had access 

 to the original paper, and that I have some doubts as to 

 the strict accuracy of some of the deductions therefrom, 

 which I have not yet had time fully to confirm. I, however, 

 hope shortly to do this with the assistance of the new gal- 

 vanometer. 



The main features of Ohm's theory are simply expressed 

 in the following formula, f = J^, in which F denotes the 

 force of the current, E the electro-motive forces, and R 

 the resistances. These are thus defined by Professor 

 Wheatstone : — 



" By electro-motive force is meant the cause which in a 

 closed circuit originates an electric current, or in an un- 



